In a continuous casting steel-making process, molten steel is poured from a ladle into a large vessel known as a tundish. The tundish has one or more outlets through which the molten steel flows into one or more respective moulds. The molten steel cools and solidifies in the moulds to form continuously cast solid lengths of metal. A submerged entry nozzle is located between the tundish and each mould, and guides molten steel flowing through it from the tundish to the mould. The submerged entry nozzle has the form of an elongate conduit and generally has the appearance of a rigid pipe or tube.
An ideal submerged entry nozzle has the following main functions. Firstly, the nozzle serves to prevent the molten steel flowing from the tundish into the mould from coming into contact with air since exposure to air would cause oxidation of the steel, which adversely affects its quality. Secondly, it is highly desirable for the nozzle to introduce the molten steel into the mould in as smooth and non-turbulent a manner as possible. This is because turbulence in the mould causes the flux on the surface of the molten steel to be dragged down into the mould (known as ‘entrainment’), thereby generating impurities in the cast steel. A third main function of a submerged entry nozzle is to introduce the molten steel into the mould in a controlled manner in order to achieve even solidified shell formation and even quality and composition of the cast steel, despite the fact that the steel solidifies most quickly in the regions closest to the mould walls.
It will be appreciated that designing and manufacturing a submerged entry nozzle which performs all of the above functions to an acceptable degree is an extremely challenging task. Not only must the nozzle be designed and manufactured to withstand the forces and temperatures associated with fast flowing molten steel, but the need for turbulence suppression combined with the need for even distribution of the molten steel in the mould create extremely complex problems in fluid dynamics.
Furthermore, it is common to introduce aluminium into the casting process in order to combine with and thereby remove any oxygen from the molten steel—since oxygen may form undesirable bubbles or voids within the cast metal. However, it is well known that the resulting alumina tends to accumulate on the inner surface—of submerged entry nozzles employed during the casting process. This build up restricts the flow of metal through the nozzle, which, in turn, affects the quality and flow of metal exiting the nozzle. In time alumina build up may eventually completely block the flow of metal thereby rendering the nozzle unusable.
It is therefore an object of the present invention to provide an improved submerged entry nozzle.